I. Field of the Invention
This invention relates generally to the field of seismic prospecting and more particularly to a method and apparatus for injecting an acoustic pulse into the earth by means of a repetitively cycled impact mass.
II. Description of the Prior Art
U.S. Pat. No. 4,284,165, issued Aug. 18, 1981 to Tom P. Airhart, et al, entitled "Acoustic Pulse Generator" describes an apparatus for generating an acoustic pulse in the earth including an impact piston which is driven downwardly by compressed gas to strike a baseplate positioned on the earth's surface. The kinetic energy of the impact piston is transformed into elastic-wave energy at the time that the piston strikes the baseplate and that elastic-wave energy pulse is transmitted into the earth for purposes of seismic prospecting. Seismic prospecting utilizes reflections of this elastic wave from the various strata beneath the surface of the earth as is well known in the art. In preparation for the next pulse generation, the impact piston is lifted from the baseplate and latched in a position some distance above it. This process can be repeated until such time as the desired seismic processing is completed.
While using the above-described apparatus for seismic prospecting, additional undesired noise in the frequency range of interest was detected which was apparently due to the retrieval of the impact piston. This undesired noise is generated by the lift rods contacting a flange attached to the impact piston and, by the relaxation of the stress on the earth-contacting baseplate, during the brief time that the load is being removed from the center of the baseplate. Attempts to eliminate this noise through the use of springs and hydraulic fluid accumulators have not generally been sufficient to permit retrieval during recording. Furthermore, attempts at filtering the noise out of the received signals have been unsuccessful. Received reflected elastic-wave signals are significantly attenuated during their transit time and must be processed in order to remove undesired noise. The filtering of this undesired noise is best accomplished when the noise itself is coherent in nature, repeatable, and occurs early in the recording cycle. The prior art method of retrieval did not provide a coherent and repeatable noise.
Therefore, impact piston retrieval has been delayed until after all desired reflected signals have been received and recorded. This is typically between six and ten seconds. In addition, the time required to retrieve the mass is about eight seconds. Therefore, current total cycle time is between fourteen and eighteen seconds resulting in reduced efficiency due to longer wait periods between impact and retrieval. If the mass could be retrieved during the recording time, the cycle time could be reduced to eight to ten seconds and efficiency nearly doubled.
Therefore, the need exists for a mass retrieval method and apparatus for an elastic pulse generator that can reliably, repetitively remove the impact piston from the baseplate as early after the impact as possible.